1. Field of the Invention
The present invention relates to a light emitting device, an illumination device, and a photo sensor. In particular, the present invention relates to a light emitting device and an illumination device which include a semiconductor laser element, and to a photo sensor used in these devices.
2. Description of Related Art
In recent years, light emitting devices that use an LED have been attracting attention because an LED has a considerably longer lifetime than that of an incandescent lamp or a fluorescent lamp. Illumination devices and the like that use an LED as a white light emitting device have already been put into practical use. However, the power per LED element is generally low and a light emitting device that is required to have high power needs to use a plurality of LED elements, thus limiting the degree of downsizing of the light emitting device. Further, light emitting devices of higher luminance are demanded for use in automobile headlights and the like.
“Luminance” herein refers to a psychophysical quantity indicating the degree of brilliance, and is obtained by dividing the luminous intensity by the area of the light source.
Light emitting devices that use a semiconductor laser are gaining attention as this type of light emitting device (see JP 07-318998 A and JP 2003-295319 A, for example).
JP 07-318998 A discloses a light emitting device that includes an infrared ray generating device and a powder optical conversion material. The infrared ray generating device emits infrared light (laser light) (semiconductor laser element or the like). The powder optical conversion material converts, when irradiated with infrared light emitted by the infrared ray generating device, the infrared light into visible light and emits the visible light.
This light emitting device uses a semiconductor laser element or a similar laser light source as the infrared ray generating device, and uses infrared light, which is laser light, to irradiate the powder optical conversion material, which is placed at the focal point of a concave mirror and converts infrared light into visible light. The light emitting device is therefore reduced in size and high in luminous intensity. Moreover, the light emitting region of the light emitting device can be miniaturized, thereby realizing the light emitting device as a point light source that has excellent light collection properties.
FIG. 13 is a diagram illustrating the structure of a light emitting device disclosed in JP 2003-295319 A. As illustrated in FIG. 13, the light emitting device disclosed in JP 2003-295319 A includes an ultraviolet ray LD element (semiconductor laser element) 1001, which is a laser diode, a collimator lens 1003, which is a collimator provided in front of the ultraviolet ray LD element 1001, an aperture 1004, which is provided in front of the collimator lens 1003, a condenser lens 1005, which is a condenser provided in front of the aperture 1004, a fluorescent substance 1006, which is provided in front of the condenser lens 1005, an ultraviolet ray reflecting mirror 1007, which is a laser light reflecting mirror provided in front of the fluorescent substance 1006, and a visible light reflecting mirror 1009, which is provided such that the condenser lens 1005, the fluorescent substance 1006, and the ultraviolet ray reflecting mirror 1007 are placed inside its parabolic reflecting surface.
In this light emitting device, laser light 1002 which is coherent light emitted from the ultraviolet ray LD element 1001 is turned into a parallel pencil of rays upon passing through the collimator lens 1003, and passes through the aperture 1004 and the condenser lens 1005 to be collected to the fluorescent substance 1006. The incidence of the laser light 1002 on the fluorescent substance 1006 causes an excitation within the fluorescent substance 1006, and the laser light 1002 is absorbed in the fluorescent substance 1006 and reduced in intensity, with the result that spontaneous emission light 1008a which is incoherent light is emitted spontaneously from the fluorescent substance 1006. Light that has not been absorbed in the fluorescent substance 1006 leaks from the fluorescent substance 1006, but is reflected by the ultraviolet ray reflecting mirror 1007 to enter the fluorescent substance 1006 again. The fluorescent substance 1006 absorbs this light and emits the spontaneous emission light 1008a. The spontaneous emission light 1008a which is incoherent light spontaneously emitted from the fluorescent substance 1006 is reflected by the visible light reflecting mirror 1009 and turned into a parallel pencil of rays 1008b, which travels in a given direction.
“Coherent light” is light that has an identical phase temporally and spatially, and high coherence.
The light emitting device disclosed in JP 07-318998 A is stated to be effective as a light emitting device for optical communication, a light emitting device for projection, a light emitting device for exposure, and the like. However, when the powder optical conversion material which converts laser light into visible light is chipped off or otherwise develops a defect for some reason, it is a risk with this light emitting device that infrared light (laser light) emitted from the infrared ray generating device which is a laser light source (semiconductor laser element or the like) exits to the outside without being converted into visible light. Laser light emitted from a semiconductor laser element is coherent light, and if exiting to the outside without being converted into visible light, may be harmful to the human eye. This problem is particularly serious when this light emitting device is used as an illumination device such as an indoor illumination device, an automobile headlight, or a searchlight.
The light emitting device disclosed in JP 2003-295319 A converts an ultraviolet ray into light that has a longer wavelength than that of the ultraviolet ray, and therefore is superior to the light emitting device of JP 07-318998 A in terms of conversion efficiency.
However, the light emitting device of JP 2003-295319 A also has a risk that a chip or other defects in the fluorescent substance 1006 cause the laser light 1002 to be reflected by the ultraviolet ray reflecting mirror 1007 and by the visible light reflecting mirror 1009 and to exit to the outside. There is another risk that the laser light 1002 exits directly to the outside when the fluorescent substance 1006 or the ultraviolet ray reflecting mirror 1007 is displaced or falls off.
Further, in the case where blue laser light which is in the visible range and beginning to be put into practical use is employed and converted into incoherent light longer in wavelength than the blue laser light, the high conversion efficiency of this light emitting device is convenient as a laser light source, but the concern for the safety of the eye is more grave when blue laser light exits to the outside without being converted into incoherent light.
With any of the light emitting devices disclosed in the cited documents, there is a risk that laser light exits to the outside while remaining coherent light and enters a human eye when the optical conversion member which converts laser light into visible light (powder optical conversion material or fluorescent substance) is chipped off or otherwise develops a defect for some reason. The disclosed light emitting devices are thus lacking in safety. In short, a light emitting device that uses laser light converted into visible light or other types of incoherent light has a risk that a trouble in the optical conversion process causes laser light to exit to the outside, which is a safety problem never encountered by conventional light emitting devices.
To make matters worse, users of these light emitting devices may not notice the problem when a chip or other defects in the optical conversion member cause laser light to exit to the outside while remaining coherent light.